Originally Posted by SuperC142

I don't have my 3VO yet and I've never used an EVO, but I'll take a stab at helping anyway because I get my mail from Yahoo (via K9 Mail on my current phone). Here are the settings I have in K9 that work for me:

INCOMING
IMAP Server: imap.mail.yahoo.com
Security Type: SSL (always)
Authentication Type: PLAIN
Port: 993

OUTGOING
SMTP Server: smtp.mail.yahoo.com
Security Type: SSL (always)
Authentication Type: PLAIN
Port: 465
Require Sign-In: [yes]

My gut tells me you might have the authentication configuration wrong (?)
Also, Complex757 (above) is correct, but also be sure to remove the leading period (i.e. "imap.mail.yahoo.com" as opposed to ".imap.mail.yahoo.com"). Hope this helps!